Fire is a complex and emotive entity. The consequences of fire are often catastrophic and disastrous. Fire destroys many seemingly indestructible objects and materials. Fire burns wood to ash, melts metals and vaporizes many other substances, often into dangerous gases. These gases are often toxic and cause severe problems, even to people trained to fight and control fires. Accordingly, it is very desirable to provide materials that are heat and fire resistant, especially in systems that enable fire fighters to carry out their jobs, for example lighting and communication systems in buildings.
Electric cables for lighting and communication systems, which are capable of operating during a fire, are becoming the standard, and often required by statute, in order to facilitate fire fighting and to limit fire propagation in buildings. Government regulations in various countries now specify that essential electrical circuits be protected in order to ensure that the electrical system be capable of operating thus assuring the safety of persons inside the building. This protection also permits firefighters to be more efficient in controlling and extinguishing fires.
Standards, such as French: NF C 32-070 ADD1 and British: BS 6387:1994, describe certification tests for electrical cables with respect to fire tolerance. These certification tests involve heating a sample of a cable, including the insulation sheath. The heating is done by an appropriate device, such as a furnace or by direct exposure to flame. During heating, the cable is energized at a rated voltage. The cable suffers a periodic mechanical shock induced, in part, by impact from a motorized arm. Failure of a cable is defined with respect to a state of fuses or breakers, which are connected in series the conductors of the cable to the power supply. The cable and wire must be able to withstand a predetermined temperature over a predetermined time in order to meet the standards.
In certain locations, such as high buildings, a minimum amount of time is needed so that all persons that may be in the building can be reached. Therefore, the electrical system during a fire must be maintained at least during that amount of time. It has been established that some essential electrical circuits must be able to operate for at least two hours, and often in excess of four hours, to ensure safety of people. Such systems include, for example, alarms which are, in turn, essential in order to enable other systems to be operated, such as telephone systems, lighting systems, elevator systems, ventilation systems, fire pumps, etc.
Electrical cables and wires used in these systems should maintain integrity and have continued conductivity performance during high temperatures that are associated with fires, at least for elongated periods of time. This will permit emergency personnel to use existing electrical systems for communications, lighting and other associated applications.
Polymeric materials, such as organic plastics and silicones, have been used as electrical insulation, for example in the insulation of the cables and wires. See for example, U.S. Pat. Nos. 5,227,586 to Beauchamp, U.S. Pat. No. 5,369,161 to Kumieda et al., 5,260,373 to Toporcer et al. While these organic materials are acceptable for their general insulation properties, the nature of organic materials in areas of fire can lead to a spread of fire, emission of smoke and release of combustion products that are dangerous to humans and injurious to equipment and human health, all of which are, of course, undesirable. Further, these insulating materials may not provide for a high temperature resistance at an elongated period of time.
Electrical insulating properties of wire and cable insulation, such as insulation formed from organic material, degrades during a fire and the high temperatures associated with a fire. The degradation of the insulation may result in failure of electrical equipment and interruption of power delivery, for example, due to electrical shorts and discharges across insulation layers. In particular, maintenance of mechanical and electrical integrity of insulation in temperatures up to about 950.degree. C. is severely degrades and impaired.
For example, many cables which are presently in use, may be capable of resisting temperatures in the neighborhood up to about 1000.degree. C. However, the insulation integrity of the wire or cable at such high temperatures is typically limited to a period of less than about 30 minutes. The insulation often fails at high temperatures over a relatively short time period. The failure results in an electrical short or electrical discharge, and thereby disables an electric supply. This is undesirable, especially in fire environments as it may prevent operation of emergency alarms and lighting systems that will assist in the evacuation of people, rescue efforts and fire fighting efforts. High temperature resistance is limited to a period of less than about 30 minutes.
Polymeric insulation based on silicone polymers with additions of both heat stabilizers and a fumed silica filler is known. However, polymeric insulation based on silicone polymers decomposes to a lower molecular weight species at temperatures above about 650.degree. C. after a relatively short time period. The decomposition of a polymeric insulation based on silicone polymers is accompanied by the evolution of water and silicon containing vapors, which is less damaging compared to caustic vapors produced by halide containing organic polymers, such as PVC. A non-volatile ash remains after decomposition. The non-volatile ash can be described as a porous glass or ceramic comprising silicon, oxygen and carbon. An x-ray diffraction of the pyrolyzed silicone ash in indicates a very fine grain size or amorphous structure. The electrical conductivity, thermal conductivity and mechanical properties of the polymeric material are largely determined by its microstructure and density, as well as exact ratios of silicon, oxygen and carbon remaining in the ash.
While polymeric insulation based on silicone polymers, such as silicone polymers, with additions of both heat stabilizers and a fumed silica filler provides adequate insulation properties for relatively low temperatures and for only short time periods, they are not generally acceptable for high temperatures and heat associated with fires, and especially for elongated time periods.